Various physiological activities of lactoferrin (hereinafter abbreviated as Lf) such as acceleration of iron absorption, prevention of lipoperoxide formation, antibacterial and antiviral activity, cell proliferation, and control of immune system have been known. Therefore, Lf has been used in the production of foods, medicines, feeds and cosmetics. These products are generally pasteurized thermally, or treated with boiled water before use.
However, Lf has an essential drawback of instability against heat. Heat treatment causes precipitation of Lf due to thermal denaturation, and loss of iron binding property which leads to a physiologically inactive state. Investigation for the stabilization of Lf against heat was carried out and revealed that heating at pH 4 does not affect the binding property of Lf to iron (Davidson and Lonnerdal. Am. J. Physiol., 257: G930-G934, 1989). However, heat treated Lf at pH 4 gradually degenerates and loses its physiological properties. Additionally, heat treated of Lf under low ionic strength maintained its physiological activity (Japanese Laid-open Patent Application No. 108629/1992). However, practical ionic strength in commonly used products is not necessarily low. The relative effect on heat stability of Lf to pH and electric conductivity, .OMEGA., was investigated to develop a method of preparation of heat resistant of Lf and showed stabilization of Lf solutions against heat by adjusting log .OMEGA. to satisfy the following equations (Japanese Laid-open Patent Application No. 8269/1992). EQU log .OMEGA..gtoreq.(2.96/pH)+0.64 (pH&lt;5) EQU log .OMEGA..gtoreq.(29.37/pH)-4.62 (5.gtoreq.pH.gtoreq.7.9) EQU log .OMEGA..gtoreq.-0.917 (pH&gt;7.9)
Lf solutions out of the ranges mentioned above require separate pasteurization of Lf solutions and the other raw material solutions followed by aseptic mixing of the pasteurized solutions. Japanese Regulations demand single pasteurization by heating after mixing, and does not allow separate pasteurization and mixing.